The present invention relates to an objective lens of an optical pickup for at least two types of optical discs requiring different NAs (numerical apertures).
Generally, recording density on an optical disc is closely related to a beam spot size formed on a data recording surface of the optical disc. That is, the beam spot size should be neither too large nor too small in view of a track width on the data recording surface. If the recording density of the data is relatively high, the track width becomes narrower, and thus, the beam spot size should also be smaller. If the recording density is relatively low, the track width is wider, and thus, the beam spot size should also be larger. If the spot size is too large with respect to the track width, information on an adjoining track may be mixed as a noise component (which is known as a cross-talk), while if the beam spot size is too small with respect to the track width, and, in particular, when a data reproducing method makes use of a diffraction of light as in a CD reading system, a sufficient diffraction effect cannot be obtained and some signals may not be reproduced under such a condition.
The size of the beam spot formed by a laser beam is smaller as the NA of the optical system is larger. If the disc is a DVD (Digital Versatile Disc) or an MO (Magneto-Optical disk), which has a relatively high recording density, the optical system is required to be the high-NA system, while if the disc is a CD whose recording density is relatively low, the optical system is required to be the low-NA system. If a common objective lens is used for both the DVD (or MO) and the CD, it is necessary to limit the diameter of the beam when the CD is used so that the NA does not become too large. For this purpose, there is known an objective lens provided with a diffraction lens structure, which functions to confine the beam for the CDs.
If the beam is confined using the diffraction lens structure when the CD is used, a ring of light (known as a side lobe) is formed. If the intensity of the side lobe is relatively strong, an S/N ratio of a reproduced signal increases.